Passive surveillance systems are widely used to detect underwater objects. These systems employ a plurality of individual signal sensors, sometimes as many as 1,000, placed at different locations surrounding the area to be monitored by the system. Because each sensor is situated at a different location, a single signal emanating from a single source will produce slightly different responses at each sensor. For example, sensors which are closer to the source will receive the signal earlier. Generally, the object of a passive surveillance system is to compare and contrast the signals received by individual sensors--looking particularly at the delays between the sensor-received signals--to determine the location of the signal source.
In an ideal environment, determination of the signal source location is a relatively straightforward process, and follows directly from well understood principles of analytic geometry and linear systems theory. Real systems, however, must contend with a variety of factors that substantially complicate the source location process. These factors include background noise (both environmental and system-generated), multi-path interference caused by reflection of the signal off the surface or bottom of the ocean, or simultaneous reception of multiple signals, to name a few.
To improve noise immunity and the ability to perform sophisticated signal-processing of the transducer-received signals, modern passive surveillance systems typically operate by (i) digitally sampling the signals at each transducer, (ii) converting each signal to the frequency domain and (iii) performing some type of statistical correlation analysis utilizing these frequency domain transducer signals. The type of correlation analysis performed varies depending upon the objective to be achieved. For example, U.S. Pat. No. 4,980,870, entitled ARRAY COMPENSATING BEAMFORMER, incorporated herein by reference, describes a passive surveillance system wherein the signals from individual transducers are mathematically combined in a manner that maximizes the reception of signals from a certain direction, and/or minimizes the reception of signals from other directions, so as to act like a highly-directional microphone pointed in the "steering direction." U.S. Pat. No. 5,099,456, entitled PASSIVE LOCATING SYSTEM, incorporated herein by reference, uses a similar frequency-domain digital sampling apparatus, but instead analyzes the individual frequency-domain signals with an aim to ascertain the location of the common source from which these signals emanate.
In all passive surveillance systems, it is assumed that the location, or at least the relative location, of each sensor is known. This sensor location information is required to interpret the sensor-received signals in a meaningful way. Once the system knows the relative location of its sensors and the velocity of signal propagation in the particular medium, the system can anticipate the time it should take for a given signal to propagate (along a particular steering direction) from one sensor to another. By comparing the times at which the signal is actually received at various sensors to the predicted times-of-arrival, the system can determine the direction from which the signal emanates.
Inaccurate information regarding the sensor locations introduces serious errors into the process. Without accurate sensor position information, the system cannot predict the expected delays between sensors along given propagation paths and, as a result, will be unable to accurately compute the signal source location. If the array is instead used in a beam-forming application, the inaccuracy in sensor locations will be reflected as reduced directional specificity in the "beam" and/or reduced detectability of the signals monitored using the beam.
Unfortunately, measuring and maintaining exact sensor locations is not as simple as might appear, at least in the underwater environment. First, the long wavelengths involved often mandate sensor arrays of a mile or more. Clearly, therefore, a fixed, rigid array structure (wherein the relative sensor locations cannot shift) is impractical, or at least highly uneconomical.
Thus, there is a need for a method and apparatus for ascertaining the relative positions of sensors in a passive sonar system, preferably without need for high signal bandwidth in the sensors.